Process for producing anticorrosive surface treated steel sheets and product thereof



1967 SHIGERU YONEZAKI ETAL 3,296,100

PROCESS FOR PRODUCING ANTICORROSIVE SURFACE TREATED STEEL SHEETS ANDPRODUCT THEREOF Filed April 29, 1963 E U C I l l I Jk 2 4 6 8 i0 l2 I4l6 S e o INVENTORS 13 wwmw, M, Pound United States Patent ()fifice3,296,100 Patented Jan. 3, 1967 PROCESS FOR PRODUCING ANTICORROSIVESURFACE TREATED STEEL SHEETS AND PRODUCT THEREOF Shigeru Yonezaki,Hajime Nitto, and Hidejiro Asano, all of Kitakyushu, Japan, assignors toYawata Iron & Steel Company Limited, Tokyo, Japan, a Japanesecorporation Filed Apr. 29, 1963, Ser. No. 276,549 Claims priority,application Japan, May 9, 1962, 37/ 18,896 3 Claims. (Cl. 204-41) Thisinvention relates to a process for producing anticorrosive steel sheetsby electrolytically treating steel sheets in a very dilute chromicanhydride-sulphuric acid bath.

Generally, in surface-treated steel sheets made by using a chromicanhydride bath, there are many defects such as, for example, poor paintadhesion, cracking and peeling due to the brittleness of the film formedon the treated steel sheets, and these defects are considered to becaused by the nonmetallic crystalline formation of the film. In order toeliminate such defects, practical electrodepositing processes have beeninvestigated. However, no satisfactory results have yet been obtained.

As a result of long research, the inventors have succeeded in completingthe present invention, by which the defects such as are mentioned abovecan be entirely eliminated.

An object of the present invention is to provide an economical processfor producing surface-treated steel sheets which have good corrosionresistance, good paint retention, and good impact resistance.

Other objects of the present invention will become clear from theaccompanying drawing and the following specification.

The drawing shows the amount of material deposited in a metallicchromium layer and that of a chemically treated film layer thereon on asteel sheet by electrodeposition according to the method of the presentinvention depending on the treating time.

The inventors have found that when a steel sheet is electrolyticallytreated in a dilute aqueous solution ott chromic anhydride of aconcentration of less than 50 g./l. and having therein a sulphuric acidradical in an amount of 0.25% by weight of the chromic anhydride from asulphate or sulphuric acid, such a surface-treated steel sheet havinggood corrosion resistance, good paint retention and good impactresistance can be produced at a low cost.

The fact that the concentration of chromic anhydride is limited so thatit is less than 50 g./l. is especially important in the presentinvention. That is to say, only when the concentration of chromicanhydride is limited so that it is less than 50 g./l., will anonmetallic film be deposited without forming nonmetallic crystals whichare considered to be the cause of the defects of steel sheetselectrolytically treated With chromic anhydride according to theconventional methods. In other Words, only in an aqueous solution ofchromic anhydride of a concentration of less than 50 g./l., does the pHvalue on the surface of the cathode vary so that metallic chromium iselectroplated thereon and, at the same time a reduced trivalent chromiumcompound is deposited on a layer of said electroplated chromium. Thus,in the electrolytic treatment according to the present invention twolayers will be electrodeposited on a steel sheet: a metallic chromiumlayer as the bottom layer and a nonmetallic chromate film therein as theouter layer. By forming these two layers surface-treated steel sheetshaving good corrosion resistance, good paint retention and good impactresistance can be obtained. If the concentration of chromic anhydride isabove 50 g./l. there will not be produced the two layers as in thepresent invention. On the other hand, if the concentration of chromiumanhydri-de is made less than 10 g./l., the consumption of electric powerwill have to increase, reducing the efficiency of the use of theelectricity and further the treatment temperature will have to bereduced, resulting in the formation of an irregular film. Thus, becausein an aqueous solution of chromic anhy-dride of a concentration of lessthan 10 g./l. such defects as above mentioned will occur, it ispreferable to make the concentration of chromic anhydride more than 10g./l. for a practical operation.

The range of the amount of chromium sulphate to be added to the abovementioned solution or sulphuric acid to be added to the same solution inan amount equivalent to the sulphuric acid radical of said sulphateshould be limited to be 0.2 to 5.0% by weight of the chromic anhydridecontained in the aqueous solution for the following reasons. If chromiumsulphate is add-ed in an amount larger than 5.0% by weight of thechromium anhydride, both the paint adherence and corrosion resistance ofthe upper nonmetallic chromate film therein will be reduced; but, if itis less than 0.2%, the amount of the sulphuric acid radical will be toosmall to attain the effect of the present invention.

The object of the present invention can also be attained by usingsulphuric acid. However, in this case, the treatment requires more timethan when chromium sulphate is used. Therefore, preparatory to theelectrolytic treatment a preelectrolytic treatment is to be carried outand the cathode is to be replaced by a new one for the subsequentelectrolytic treatment. The quantity of electricity for saidpreelectrolytic treatment is not critical but, it may be more than 8,000coulombs/l. by a current density of 10 A./dm.

The lower the bath temperature, the better the current effect-and thethicker the film formed. But, as it is necessary to cool theelectrolytic bath and the luster of the film will also decrease, atreating temperature in a range of about 15 to 50 C. is recommended.

Further, the higher the current density, the higher the currentefiiciency and the higher the corrosion resistance. But, in such case,the color of the film will become blackish and the impact resistanceafter painting will be likely to be reduced. Therefore, it is necessaryto properly select the current density in accordance with the use to bemade of the product.

The figure is a diagram showing the relation between the total amount ofchromium deposited on the treated steel plate (mg/rim?) and the treatingtime in seconds at 40 g./l. of CrO 0.542 g./l. of Cr (SO (1.35% of theCrO a bath temperature of 40 C. and a current density of 20 A./dm. Inthe figure, (a) shows an electro deposited layer of metallic Cr as abottom layer formed on the sample steel sheet, (b) shows a chemicallytreated coating of nonmetallic chromate compound therein formed on theabove mentioned metallic Cr and shows a total amount of Cr.

If a steel sheet is treated so as to satisfy such re quirements as arementioned above, the corrosion resistance (as tested in accordance withJ.I.S.Z. 2371 (and the paint retention of the steel sheet will be as inTable 1.

TABLE I.ANTICORROSIVENESS AND PAINTABILITY Paintability of 1 finish-Paintability and 2 lacing paint (DuPont Salt water spray (forquerability of inner Steel sheets treated under the above mentionedconditions were subjected to salt water spray tests by spraying asolution of 5% NaCl at 35 C. at 20 pounds per square inch, Du Pont typepaint adhesion and impact tests, cross-cut tests and drawing tests. Theresults of these tests are shown in Table 2.

were subsequently electrolytically treated for 5 seconds under the sameconditions as in Example 1, with the exception that the cathode usedduring the pre-electrolytic treatment was replaced with new one. Then,the same favorable results as in Table 2 were obtained.

What is claimed is:

1. A process for producing surface-treated anticorrosive steel sheet,comprising immersing a cathode in a dilute aqueous treating solutionconsisting essentially of 10- 15 g./l. chromic anhydride and a sulphuricacid radical in an amount of 0.2 to 5% by weight of the chromicanhydride in said solution, preliminarily electrolyzing said solutionwith said cathode to form trivalent chromium ions in an amount of 0.2 to5% by weight of chromic anhydride in the treating solution, thenreplacing the cathode with said steel sheet to be surface treated, andthen electrolyzing said solution further, whereby an intermediate layerof metallic chromium is first formed on the surface of said steel sheetand a chemically treated layer of non-metallic chromate is then formedthereon.

2. A process for producing surface-treated anticorrosive steel sheet,comprising immersing a steel sheet in a dilute aqueous treating solutionconsisting essentially of 10-15 g./l. chromic anhydride and a sulphuricacid radical and trivalent chromium ions each in an amount of 0.2 to 5%by weight of the chromic anhydride in said solution, and electrolyzingsaid solution with said steel sheet as a cathode, whereby anintermediate layer of Notes: The product (A) of the method of thepresent invention was the product in Exmetallic chromium is first formedon the surface of steel sheet and a chemically treated layer ofnon-metallic chromate is then formed thereon.

3. A surface-treated anticorrosive steel sheet having a base of steel,and an intermediate layer of metallic chromium and a surface layer ofchromate formed on the steel base by the process comprising immersing asteel sheet in a dilute aqueous treating solution consisting essentiallyof 10-50 g./l. chromic anhydride and a sulphuric acid radical andtrivalent chromium ions each in an amount of 0.2 to 5% by weight of thechromic anhydride in said solution, and electrolyzing said solution withsaid ample 1. The product (B) of the same was a product obtained underthe conditions of Example 1 when the current density was 20 A./dm.

Example 2 Chromic anhydride g./l 20 Chromium sulphate g./l 0.2Temperature C. 30 Current density A./dm. 15 Time seconds 5 Example 3Chromic anhydride g./l 10 Chromium sulphate g./l 0.1 Temperature C. 20Current density A./dm. 10 Time seconds 10 Example 4 Chromic anhydrideg./1 40 Sulphruic acid g./l 0.42 Temperature C 35 Quantity ofelectricity in preparatory electrolysis coulombs/l 8,000

After steel sheets were subjected to the preelectrolytical treatmentunder the above mentioned conditions, they steel sheet as a cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,177,392 10/1938 Mardick 20451 2,998,361 8/1961 Kitamura 20456 3,032,487 5/1962Yonezaki et al. 20456 3,081,238 3/1963 Gurry 204-56 X 3,113,845 12/1963Uchida et al. 20441 X OTHER REFERENCES Haring, H. E. et al.;Electrodeposition of Chromium from Chromic Acid Baths, Dept. ofCommerce, Natl Bureau of Standards, pp. 426-427, 1927.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, G. KAPLAN,

Assistant Examiners.

EST AVAILABLE COPY UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent. No 3,296 100 January 3, 1967 Shigeru Yonezaki et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 4 lines 10 and 24, for "15'.', each occurrence, read 50 Slgnedand sealed this 26th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

2. A PROCESS FOR PRODUCING SURFACE-TREATED ANTICORROSIVE STEEL SHEET,COMPRISING IMMERSING A STEEL SHEET IN A DILUTE AQUEOUS TREATING SOLUTIONCONSISTING ESSENTIALLY OF 10-15 G./L. CHROMIC ANHYDRIDE AND A SULPHURICACID RADICAL AND TRIVALENT CHROMIUM ANHYDRIDE AND A SULPHURIC ACIDRADICAL AND TRIVALENT CHROMIUM IONS EACH IN AN AMOUNT OF 0.2 TO 5% BYWEIGHT OF THE CHROMIC ANHYDRIDE IN SAID SOLUTION, AND ELECTROLYZING SAIDSOLUTION WITH SAID STEEL SHEET AS A CATHODE, WHEREBY AN INTERMEDIATELAYER OF METALLIC CHROMIUM IS FIRST FORMED ON THE SURFACE OF STEEL SHEETAND A CHEMICALLY TREATED LAYER OF NON-METALLIC CHROMATE IS THEN FORMEDTHEREON.